This is truly mind-boggling. It's the annual Valparaiso Cerro Abajo downhill race for mountain bikes, held each year in Chile's third-largest city.
The first video clip, from the 2010 race, shows the course from the perspective of a rider, filmed through a helmet camera. Watch for the dog that gets in his way when he's at high speed . . .
The next clip, from the 2008 race, shows the course from the perspective of spectators. It gives you the other side of the coin from the riders' point of view.
I don't think I want to be anywhere near that course during the annual race! How they avoid multiple deaths, maimings, mutilations and lacerations, I just don't know . . .